As it is known, pelletizing is an agglomeration process whereby the fine iron ore particles are converted into spherical bodies with size ranging from 8 to 18 mm. These spherical bodies having an appropriate physical, chemical and metallurgical properties for use in steel mill's reduction reactors.
The quality of the product yielded by the pelletizing process has direct relation with the material size fed to the process, and it is suitable that at least 50% of the raw material charged to the process have a size less than 0.40 mm (325 mesh) and a specific surface of 1200 cm.sup.2 /g. Usually, the ore fines employed in pelleting have a granulometry 100% less than 0.149 mm (100 mesh), however only 30 to 45% being less than 0.044 mm (325 mesh). Therefore, in order to adapt the granulometric characteristics and specific surface of the ore fines to the pelleting process requirements it becomes necessary to submit the ore fines to a crushing stage to reduce the particles size.
In the conventional pelletizing process the reducing of size (crushing and grinding) of the ore fines is achieved by grinding in a tubular mill wherein steel balls or truncated steel cones (cylpebs) are usually employed to help with the grinding operation or as a grinding medium. This grinding operation is a heavy burden on the overall production cost of pellets due to the significant consumption of energy and grinding media.
There are several options available to the grinding operation. It can be performed in or wet or dry, open or closed operations.
In the wet grinding process iron ore and water are mixed together and both are added to the mill in adjusted proportions performing a diluted grounded ore pulp. A large amount of water that is added to the grinding operation is removed by subsequent thickening, homogenizing and filtering stages.
On the other hand, dry grinding operation requires the prior drying of the ore fines, however disregards the water draining step that is necessary when the wet process is employed.
Grinding in an open operation consists in passing the material through the mill only once, while in the closed operation the hydrocyclones employed for wet grinding or the air classifiers used in the dry grinding process perform the granulometric classification at the discharge of the mill.
Finally, in the closed mills a fraction of sufficiently fine material below 0.44 mm (325 mesh) goes on to the next stages of the pelletizing process while the coarser fraction returns to the mill as the circulating load of the grinding process.